1. Field of the Invention
The invention generally relates to electromagnetic shields, and more particularly, to a multi-layer shield for absorbing and dissipating electromagnetic energy within dam processing equipment and the like.
2. Related Art
Most countries have international standards that set an acceptable limit for spurious transmission of electromagnetic (EM) energy from data processing equipment, such as radio transmitters and the like. These standards vary from country to country. The European standard for instance, is much more stringent than the American standard. Thus, a goal of transmitter manufactures is to produce electromagnetically "quiet" devices that exceed domestic standards so that their devices can be marketed abroad.
Conventional transmitter EM shielding typically reflects some amount of the energy back towards the source. For example, a radio frequency (RF) transmitter having a housing, an oscillator and amplifier circuit located in the housing, generates RF power for transmission of signals. EM energy is radiated at a certain wavelength within the housing. These waves usually encounter a shield either located around the oscillator and amplifier circuit or adjacent the inner surface of the housing. The shield grounds some of the energy and reflects some of the energy back towards the source. The reflected energy can cause standing wave patterns within the housing causing phase and/or amplitude interference with the source and degradation in RF performance, particularly, power output and inner-modulation distortion.
What is desired is an EM shield that absorbs spurious transmissions within the transmitter housing, or the like, and substantially prevents EM energy from being reflected back to the source.